Various medical devices can non-invasively measure parameters of blood in a patient. Pulse oximetry devices are one such non-invasive measurement device, typically employing solid-state lighting elements, such as light-emitting diodes (LEDs) or solid state lasers, to introduce light into the tissue of a patient. The light is then detected to generate a photoplethysmogram (PPG). These photoplethysmography systems can also measure changes in blood volume of tissue of a patient and calculate various parameters such as heart rate, respiration rate, and oxygen saturation.
However, some conventional optical pulse oximetry devices only measure certain limited blood parameters, and lack the ability to measure other patient physiological parameters. Some optical pulse oximetry devices are also subject to patient-specific noise and inconsistencies which limit the accuracy of such devices. For example, monitoring infants or patients in intensive care units can be difficult. Motion of the patient and other incidental factors can lead to noise and inaccuracies of optical-based measurements. Some optical measurement systems are sensitive to shifts in venous blood volumes, introducing errors into arterial blood measurements.
Capacitive sensing has been employed to measure some physiological parameters by applying electric fields directly through tissue using two-plate capacitors having individual plates positioned on different sides of the tissue. This two-plate capacitive sensing can be combined with optical measurement to determine changes in volume of tissue due to cardiac pulsing.